Issue 0CFP.12: P1: Zero payloads and set payload function

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: WG14, Jim Thomas
Date: 2017-03-04
Reference document: N2125
Submitted against: Floating-point TS 18661 (C11 version, 2014-2016)
Status: Fixed
Fixed in: C23
Converted from: n2397.htm

Summary

This is about an issue raised by Joseph Myers in SC22WG14.14450:

The specification for setpayload (and likewise setpayloadsig) says "If pl is not a positive floating-point integer representing a valid payload, *res is set to positive zero."

Does "positive" as applied to "floating-point integer" here mean "with sign bit 0" (the list of definitions in IEEE 754 doesn't include "positive")?  In the preferred encodings for binary interchange formats, 0 is a valid payload for quiet NaNs.  So should +0.0 as an argument to setpayload result in a quiet NaN with payload 0, while -0.0 results in *res being set to +0.0 because -0.0 isn't positive (and for setpayloadsig, both result in *res set to +0.0 because a payload for a signaling NaN has to be nonzero to avoid all mantissa bits being zero)?

A “positive floating-point integer” is a positive integer in the floating-point format, hence it is greater than zero. So, the current specification for setpayload and setpayloadsig is flawed in that it doesn’t allow setting the payload to zero.

A more basic problem is that TS 18661-1 assumes IEC 60559 interprets payloads as integers. This is true for decimal formats. IEC 60559 says:

The payload corresponds to the significand of finite numbers, interpreted as an integer with a maximum value of 10^(3×J)−1, …

The significand c interpreted as an integer is assumed throughout to be non-negative, while the s field in (s, q, c) provides the sign. For decimal, interpreting the bits in the encodings allows the two encoding schemes to have the same payloads and the payloads to fit conceptually with their encoding schemes.

However, for binary formats, IEC 60559 says:

For binary formats, the payload is encoded in the p−2 least significant bits of the trailing significand field.

Nowhere does it actually interpret the payload for binary formats as an integer.

However, the payload for binary formats has a natural interpretation as an unsigned integer, so it is reasonable for TS 1866-1 to interpret payloads (for binary and decimal formats) as such.

The suggested Technical Corrigendum below addresses these problems.

Suggested Technical Corrigendum

In 14.10, replace the first sentence:

IEC 60559 defines the payload of a NaN to be a certain part of the NaN’s significand interpreted as an integer.

with:

IEC 60559 defines the payload of a NaN to be a certain part of the NaN’s significand. The payload can be interpreted as an unsigned integer.

In 14.10, in the new C subclause F.10.13, replace:

IEC 60559 defines the payload of a quiet or signaling NaN as an integer value encoded in the significand.

with:

IEC 60559 defines the payload of a quiet or signaling NaN as information encoded in part of the NaN significand. The payload can be interpreted as an unsigned integer.

In 14.10, in the new C subclauses F.10.13.2#2 and F.10.13.3#2, change:

If pl is not a positive floating-point integer representing a valid payload, *res is set to positive zero.

to:

If pl is not a floating-point integer representing a valid payload, *res is set to positive zero.

Comment from WG14 on 2018-10-18:

Apr 2017 meeting

Committee Discussion

The committee agrees that this is a defect and accepts the Suggested Technical Corrigendum

Proposed Change

In 14.10, replace the first sentence:

IEC 60559 defines the payload of a NaN to be a certain part of the NaN’s significand interpreted as an integer.

with:

IEC 60559 defines the payload of a NaN to be a certain part of the NaN’s significand. The payload can be interpreted as an unsigned integer.

In 14.10, in the new C subclause F.10.13, replace:

IEC 60559 defines the payload of a quiet or signaling NaN as an integer value encoded in the significand.

with:

IEC 60559 defines the payload of a quiet or signaling NaN as information encoded in part of the NaN significand. The payload can be interpreted as an unsigned integer.

In 14.10, in the new C subclauses F.10.13.2#2 and F.10.13.3#2, change:

If pl is not a positive floating-point integer representing a valid payload, *res is set to positive zero.

to:

If pl is not a floating-point integer representing a valid payload, *res is set to positive zero.